This week, PVL MSc Conor Hayes considers the Martian Ocean hypothesis. This theory has several strong lines of evidence in its favour. However, some of the expected geological remnants, such as a clear shoreline lying along a geopotential, are lacking. Above: features described by some authors as putative "mega-tsunami" backwash channels on Mars, perhaps caused by a meteor impacting a putative ancient Martian ocean. (NASA/JPL/University of Arizona)
by Conor Hayes
Although it is pretty clear that liquid water once existed on the surface of Mars, there is still ongoing debate over how much water was present, as well as how long it lasted. One of the more exciting theories is the “Mars ocean hypothesis,” which posits that the planet’s northern hemisphere hosted a large ocean that covered about a third of its surface. If you look at a terrain map of Mars, this theory intuitively makes sense. Much of the northern hemisphere is a large basin, about five kilometers below the average terrain elevation (sometimes called the datum) and comparatively lacking in features like impact craters. The distribution of former stream channels and river deltas also appears to be consistent with the idea of these primordial rivers flowing into an ocean. The presence of such a large body of water would have significant implications for the potential habitability of Mars in the past, particularly given the thicker atmosphere and higher temperatures needed to sustain that volume of liquid water for an extended period of time.
One problem facing this theory is the lack of an obvious shoreline. Although several potential shorelines have been identified, none have been particularly convincing. In addition to having alternate geological explanations, these proposed shorelines show substantial changes in elevation along their lengths, on the order of several kilometers. Because water likes to lie flat along gravitational potentials, this would suggest either that some kind of geological rearrangement occurred between the formation of the shorelines and the present day or, more likely, that these features aren’t shorelines at all.
In addition to providing clear direct evidence for a Martian ocean, finding shorelines would also help us constrain the properties of Mars’ early atmosphere. Here on Earth, shorelines are largely cut by wind-driven waves, in addition to other phenomena like tides. If ancient shorelines do exist on Mars, then that necessarily implies that the atmosphere was once dense enough to allow the wind to form significant waves. Conversely, the lack of shorelines does not necessarily imply the lack of an ocean, but rather suggests that the atmosphere may have been too thin for wave formation.
Interestingly, the atmospheric pressure required for winds similar to those observed on Mars today to generate ocean waves is much lower than on Earth. This is because gravity plays an important role in the behaviour of fluids. In a lower-gravity environment, like that found on Mars, waves form much more easily at a given surface pressure. A pressure of 50 millibars (about 5% that of sea-level pressure on Earth) would require winds of 30 kilometers an hour to form waves, while an Earth-like atmosphere on Mars could sustain waves with a wind speed of only five kilometers an hour.
In 2016, another theory was put forward to explain the missing shorelines: massive tsunamis caused by two meteor impacts. The authors of this theory present evidence of extant backwash channels, formed when the ocean suddenly rushed inland before slowly draining back out. These would have been dramatic events, as evidenced by maximum inland run-up distances of 500+ kilometers that would have required typical wave heights of 50 meters, possibly up to 120 meters in some areas. Such violent events would have obliterated much of the existing shoreline, resulting in the situation we see today.
The Mars ocean hypothesis has a number of other problems that it must address. For example, we would expect that a Martian ocean would undergo a carbon cycle much like Earth’s oceans do, perhaps even to a greater extent due to the higher concentration of carbon dioxide in Mars’ atmosphere. This process would have resulted in the deposition of carbonate minerals on the ocean floor, something that we have not yet observed in meaningful amounts. One could explain this discrepancy by making the ocean more acidic, which would inhibit carbonate formation.
Regardless of whether or not Mars did have an ocean on its surface at some point in the past, it’s still fun to think about sitting on a Martian beach in your spacesuit, watching as a gentle breeze stirs up large, slow-moving waves that break against the shoreline with less force than you might expect given their size. When I read about things like this, I am reminded why I decided to study astronomy in the first place. The universe is a big place full of sights that can be simultaneously familiar and entirely alien, and you don’t even need to go far from home to experience them. True, Mars may not have oceans now, but being able to explore the ghosts of what once was is perhaps just as awe-inspiring as seeing those ancient waves myself.
Sources:
Rodriguez et al. 2016 (https://doi.org/10.1038/srep25106)
Banfield et al. 2015 (https://doi.org/10.1016/j.icarus.2014.12.001)
Fairén et al. 2004 (https://doi.org/10.1038/nature02911)
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